In recent years, because of the global need for lower fuel consumption and less exhaust emission, electric and hybrid vehicles mounted with an AC motor as a mechanical power source are proposed. By way of example, a hybrid vehicle is mounted with a DC power supply, which may be a secondary battery, an electric power converter, which includes inverters, and AC motors. The power supply is connected to the motors via the power converter. The inverters convert the DC voltage from the power supply into AC voltages for driving the motors.
The hybrid or electric vehicle is provided with a control apparatus, which includes two or three current sensors for sensing the currents in two phases or three phases of a three-phase motor. The apparatus controls the motor according to the outputs (sensed currents) from the current sensors. Thus, it is necessary to provide two or three current sensors for each of motors. This results in a bar to simplification of the parts of the inverter that include three-phase output terminals. This also results in a bar to reduction of the cost of a motor control system of the vehicle.
Patent document 1 (JP-A-2010-124544, US 2010/0123418) and patent document 2 (JP-A-2004-159391) disclose improved control apparatuses.
Specifically, patent document 1 discloses a technique effective for improving the vehicle fuel consumption rate in an AC motor control apparatus. This technique is a method for driving the AC motor by means of square wave control (one pulse switching in one current supply cycle period). The rectangular wave control is the torque feedback control that includes: calculating, without current commands for the d-axis and q-axis, an estimated torque from a d-axis current and a q-axis current that are sensed by current sensors; and so controlling the voltage vector phase that the estimated torque follows up a command torque. The rectangular wave control is adopted in operating ranges where a weak field current needs to flow. This control is effective in improving the fuel consumption rate (reducing the inverter loss) because the control makes it possible to minimize the weak field current and the number of times the inverters are switched.
Patent document 2 discloses a technique for lowering the cost of production by reducing the number of current sensors. This technique includes: providing a current sensor for sensing the current in one phase (for example, U-phase) of an AC motor; and calculating estimated currents in the other two phases (for example, V-phase and W-phase) from the sensed current, a d-axis command current, a q-axis command current, and an electrical angle of the motor.
Specifically, this technique includes: determining a U-phase current phase angle θ′ (=θ+α) by adding the command current phase angle α between the q-axis and the vector resultant from the d-axis command current Id* and q-axis command current Iq* of the AC motor to the angle θ between the rotor of the motor and the U-phase axis of the stator of the motor; calculating a current amplitude Ia from the U-phase current phase angle θ′ and the sensed current Iu in the U-phase according to the following equation (A); calculating estimated currents Iv and Iw in the V-phase and W-phase respectively from the current amplitude Ia and U-phase current phase angle θ′ according to the following equations (B) and (C); calculating an estimated d-axis current Id and an estimated q-axis current Iq from the sensed current Iu in the U-phase and the estimated currents Iv and Iw in the V-phase and W-phase respectively; and performing the feedback control of the current in the AC motor by so calculating a command voltage for the motor that the estimated currents Id and Iq equal the command current Id* and Iq* respectively.Ia=Iu/[√{square root over (1/3)}×{−sin(θ′)}]  (A)Iv=√{square root over ((1/3))}×Ia×{−sin(θ′+120°)}  (B)Iw=√{square root over ((1/3))}×Ia×{−sin(θ′+240°)}  (C)
A control apparatus for an AC motor for driving an electric or hybrid vehicle requires rectangular wave control as disclosed in patent document 1. However, because this control apparatus does not have d-axis and q-axis current commands, it is impossible to apply to the control apparatus a method of using d-axis and q-axis current commands as disclosed in patent document 2.